The present invention relates to an improved method for carrying out froth flotation separation.
A variety of methods for purifying liquids by flotation techniques are well known. Particular applications of flotation include removing finely divided solids or organic liquid impurities from water. Flotation methods are also widely used for separating a desired mineral from solid waste materials in suspension in liquids such as water.
Flotation purification techniques typically involve forming a froth by mixing air or another gas with water to be purified or with another liquid, usually after addition of a frothing agent or other reagent to aid in froth formation or wetting. The froth formed floats on the surface of the liquid and is easily separated by mechanical means from the liquid. Impurities contained in the water or other liquid adhere selectively to bubbles of air or other gas rising through the liquid. The impurities are floated into the froth layer above the liquid and are separated from the liquid with the froth. Flotation purification of water is used commercially for removing liquid oils from water, as well as for removing various solids.
Flotation separation of liquid or particulate solid material from a liquid to be purified is often undertaken in a series of three or more stages or steps. Each step involves, first, mixing a gas with the liquid to form a froth which contains a material to be separated from the liquid, and, second, separating the froth from the liquid. After each stage is finished (except the last), the liquid is passed on to a downstream stage, where the froth formation and separation operation is repeated. The number of stages used in any given commercial embodiment of a froth flotation system depends on the difficulty of separating the particular material involved; often 4-6 stages are used.
One commercially available flotation system includes four serial stages. Liquid recovered from the last stage is recycled to the first stage, and to the intermediate second and third stages, and also to the last stage. The recycled, previously treated liquid recovered from the last stage effluent is used to mix air with the liquid in each stage by jetting recycled liquid into which gas has been induced into a body of the liquid to be treated. In each stage, the resulting froth or foam is skimmed off the body of liquid. After separation of froth, the liquid is passed to the next stage. The body of liquid being treated in a given stage is maintained substantially separate from the body of liquid in upstream or downstream stages adjacent the given stage by a baffle, which prevents froth from the given stage from migrating to upstream or downstream stages.
U.S. Pat. No. 3,647,069 discloses apparatus and a method for performing flotation separation, particularly in the setting of separating solids, such as silica, and petroleum liquid materials from water, and also for deaerating water. U.S. Pat. No. 3,864,251 discloses the use of froth flotation for recovering bitumen from tar sand and water. U.S. Pat. No. 3,884,803 discloses the use of froth flotation to purify water which has been contaminated with a high API gravity oil. The Encyclopedia of Chemical Technology, Second Edition, Vol. 9, pages 380-398, includes a discussion of the use of froth flotation for separating minerals, such as metal ores, from waste materials and water.